The high speed downlink packet access (HSDPA) is a packet transmission method of IMT-2000 and is currently employed to implement a high-speed, high-capacity downlink. In HSDPA, scheduling is performed to allocate channel resources preferentially to users in good channel conditions and thereby to improve the throughput. For this purpose, a shared control channel (HS-SCCH) for reporting scheduling information and transmission schemes is provided along with a shared data channel (HS-DSCH). Because the scheduling information differs depending on the user, it is necessary to provide the same number of shared control channels (HS-SCCH) as the number of concurrent users. HSDPA is described, for example, in non-patent document 1.
Generally, in data communications, packets are not necessarily transmitted in real time and the sizes of the packets are comparatively large. Therefore, the number of concurrent users and the number of shared control channels (HS-SCCH) can be made comparatively small. If the number of shared control channels is small, a large share of the total transmission power of a base station can be allocated to the shared data channel. Thus, the current HSDPA scheme is preferable for efficient data communications.
Meanwhile, packets in wireless communications include not only data packets but also voice packets sent by voice over internet protocol (VoIP). Voice packets are preferably transmitted in real time and their sizes are comparatively small. Therefore, to efficiently use frequency and time resources, it is preferable to multiplex and thereby simultaneously transmit packets of many users. However, according to the current HSDPA scheme, the number of shared control channels (HS-SCCH) required increases as the number of concurrent users increases. This in turn increases the share of the total transmission power of the base station to be allocated to shared control channels and therefore decreases the power usable for the shared data channel. As a result, the throughput of the shared data channel in multiplex transmission of voice packets may be reduced.
A method disclosed in non-patent document 2 tries to solve this problem. In the disclosed method, mobile stations in similar communication conditions are grouped and a common transmission scheme (including a modulation scheme, a channel coding rate, a retransmission control scheme, etc.) is used for each group. Each mobile station is able to know the corresponding transmission scheme by just identifying the group to which it belongs. This in turn makes it possible to reduce the information content as well as the number of control channels. Also, a method disclosed in non-patent document 3 proposes a new signal format that requires no control channel like HS-SCCH.
[Non-patent document 1] 3GPP, TS25.308
[Non-patent document 2] 3GPP, R2-052994
[Non-patent document 3] 3GPP, R2-052980